Sighting-in a scope
Don't get carried away!
by B.B. Pelletier
Sighting-in a scope must be frightening to many shooters because, of all the technical questions we get, a large percentage deal with problem scopes. Many of the problems can be traced to the fact that they haven't been sighted-in correctly.
Problem 1. Sight-in distance IS NOT YOUR CHOICE!
You can choose any distance you like to sight-in your scope, so what can I be saying? Just this - yes you CAN choose ANY distance at which to sight-in, but you aren't going to like more than a very limited selection of distances. You will be frustrated if you sight-in at ALL OTHER distances, then I will get a confused comment like this:
B.B. I sighted-in my rifle to hit dead-on at 40 yards, but at every other distance the pellet shoots below the aim point! I can understand when it does that at ranges farther than 40 yards, but why does it also shoot lower at 20 yards? CONFUSED
Your gun shoots lower at all other distances because THAT'S THE WAY YOU SIGHTED IT IN!! If you want your car to stop within 25 feet of applying the brakes, don't slam them on at 70 mph!
The laws of physics are more unforgiving than the laws of man.
When your pellet leaves the muzzle, IT IMMEDIATELY STARTS FALLING TOWARD THE GROUND! It doesn't rise up above the bore and then fall, like the drawings seem to show. It falls instantly, and it falls at the same rate as a pellet dropped from the same height as the muzzle. If the bore is parallel to the earth and the shot and dropped pellets both take off at the same time, they will both hit the ground at the same time. The shot pellet will hit some distance from the gun because of its velocity.
To compensate for the drop of the pellet when we sight-in, we point our scopes slightly down toward the ground, so the exiting pellet will SEEM to rise above the line of sight. That is why we speak of TWO distances at which the pellet will be dead-on with the crosshairs. I discussed this in an earlier posting about sight-in distances (At what range should you zero your scope?).
Here is where physics steps in. Those two distances are determined by the velocity of the pellet AND the rate at which it slows down because of drag. With modern adult airguns all shooting at pretty similar velocities (750-950 f.p.s.) and with diabolo pellets being so similar, the choices of aim points is limited - IF YOU WANT TWO AIM POINTS, THAT IS.
If you insist on sighting-in at 40 yards, go ahead. But don't ask me why the pellet shoots lower at all other distances. You have selected the spot in the trajectory curve where that will happen. Actually, given a spread of velocities, there is a short span of distances, all hovering around 37-45 yards, at which this will happen.
Yeah, well, B.B., I want to sight-in at six yards, because that's the distance from my back door to the garbage cans, and we have problems with raccoons. Now, can you tell me why my pellet is so much higher at all other distances? I mean, until I get way out past 50 yards, my pellet is in orbit! Please tell me how to get it back on target at 30 yards, because that's where the bird feeder is.
[B.B. has left the building - escorted by two nice men in white coats.]